1. Field of the Invention
The present invention relates to a push button switch for use as a key switch for a data input apparatus for a personal computer, a word processor or the like, and, more particularly, to a push button switch the thickness of which can be easily reduced.
2. Related Art Statement
A conventional push button switch of the type described above will be described with reference to FIG. 13.
Referring to the drawing, reference numeral 1 represents a reinforcing plate made of metal or the like. A membrane switch 2, comprising an upper sheet 2b having a movable contact 2a and a lower sheet 2d having a fixed contact 2c, is placed on the reinforcing plate 1. A case 3 is placed on the membrane switch 2, the case 3 having an annular first projecting portion 3a and a second projecting portion 3b. An operation member 4a of a key top 4 is movably positioned along an inner surface 3c of the first projecting portion 3a. the operation member 4a has a fastening claw at the lower end portion thereof so that the fastening claw is fitted within a recessed portion (omitted from illustration) formed in the first projecting portion 3a. As a result, the fastening claw can be moved within the recessed portion and the operation member 4a can be vertically moved along the inner surface 3c. Furthermore, a coil spring 5 is interposed between a flat surface 3d of the case 3 and the lower surface of the key top 4 in such a manner that the coil spring 5 is positioned around the first projecting portion 3a. In addition, another coil spring 6 is positioned in the operation member 4a by pressfitting for the purpose of pressing the movable contact 2a of the membrane switch 2. Although omitted from the illustration in FIG. 13, a recessed portion is formed in the second projecting portion 3b in a direction perpendicular to the direction of the drawing sheet for FIG. 13. As a result, the upward separation of the key top 4 is prevented by fastening the fastening claw provided for the key top 4 to the recessed portion.
Thus, when the key top 4 is depressed against the urging force of the coil spring 6, the outer portion of the operation member 4a is downwards moved along the inner surface 3c. As a result, the lower end portion of the coil spring 5 press-fitted in the operation member 4a presses the upper sheet 2b of the membrane switch 2, causing the movable contact 2a to be brought into contact with the fixed contact 2b. Therefore, the switch is switched on. When the pressure applied to the key top 4 is then released, the original state can be restored by the elastic restoring force of the coil spring 6.
Recently, there has been a desire for a compact keyboard having a reduced thickness, causing a necessity for reducing the thickness of the push button switch to arise.
When a push button switch having a reduced thickness is constituted, the push button switch must have a certain depressing stroke (3 to 4 mm).
It is assumed that the thickness of the key top 4 is b, the distance of the movement of the key top 4 is S, the length of a portion (omitted from illustration) for fastening the case 3 and the key top 4 is a, the thickness of the reinforcing plate 1 and the membrane switch 2 is c and the overall height is expressed by H.
Then, the length a of the fastening portion can be expressed by a=H-(2S+b+c). In this case, the thickness b of the key top 4, the movement distance S of the key top 4 and the thickness c of the membrane switch 2 and the reinforcing plate 1 become substantially constant depending upon the molding condition and the parts composition. Therefore, there has conventionally been a necessity for the length a of the fastening portion to be shortened at the time of realizing the above-described thickness reduction.
However, if the length a of the fastening portion is shortened, the lower end portion of the operation member 4a of the key top 4 is caught by the inner surface of the first projecting wall 3a of the case 1 when the end portion A of the key top 4 is depressed. As a result, the conventional push button switch cannot be depressed smoothly.